Several authors have reported a phenomenon that I have also experienced. In the middle of historical research on something that you find exciting and engaging, it is possible to become seized with a peculiar dread that someone else has seen this and written about it, and that you are busily researching the obvious. A related concern is one I addressed in an earlier post, that another scholar will beat you to your own results. A competitive, paranoid nervousness can set it, accompanied by twitches and strange thoughts.
[As background, I've been up to my dusty eyebrows in library materials about an 18th-century subject that I'm planning as a book. Right now I'm deep in the research stage for the sample chapter that will go out with a book proposal (in this case a hybrid between a scholarly prospectus and a trade nonfiction proposal) to potential university press publishers. I'm noting progress on this blog in the hope that others will also share their book writing experiences and chime in.]
In the middle of all this, the odd dread came upon me, and I had to sit down on the floor of the library stacks to get over it. Perhaps it was the fumes from all of those old books (I'm working with some of the oldest that are still on our shelves in the hope of finding traces of my author mentioned in works by others, with many of them pointing to books and documents that no longer exist except as reference notes), but I had a vision of turning a page or completing a computer search and seeing my own words under somebody else's name, but not plagiarized, because the article had been written years ago! Of course it was all a fantasy, but it had the effect of reality... I had to step outside for some fresh air and a reminder that nothing is new, it's what the researcher brings to the study that matters most. Most of us strive for originality and unique contributions, but we so often find traces of others in the scholarly past, laboring quietly in a solitude now covered over by time and quaint obscurity. I just know that I have a 19th-century intellectual doppelganger who touched the same pages, took similar notes, and drew startlingly like conclusions about our shared 18th-century interest. I just haven't found her or him yet. If I do, I'll simply travel back in time, crying "Thought thief!"